Bio-oils of varying properties and compositions are obtained using numerous methods and processes. Bio-oils may be obtained for example from biomass using any suitable thermal treatment, pyrolysis and the like.
Pyrolysis is generally understood as the chemical decomposition of organic materials by heating in the absence or with limited supply of oxidizing agent such as air or oxygen. Pyrolysis can be used for converting biomass to pyrolysis oil which is an example of bio-oil. Commercial pyrolysis applications are typically either focused on the production of charcoal (slow pyrolysis) or production of liquid products (fast pyrolysis), the pyrolysis oil. Both the slow pyrolysis and the fast pyrolysis processes may be used for the manufacture of pyrolysis oil.
During pyrolysis of biomass, for example of lignocellulosic material, carried out at temperatures in the range 400-700° C., most of the cellulose and hemicellulose and part of lignin typically disintegrate to form smaller and lighter molecules which are vapors at the pyrolysis temperatures. During cooling some of the vapors condense forming a liquid product, called pyrolysis oil.
Bio-oils are complex mixtures of chemical compounds, including reactive aldehydes and ketones. Said reactive compounds react with each other whereby complex molecules having higher molecular weight are formed and the viscosity of bio-oil is increased. For example biomass derived pyrolysis oil typically comprises water, light volatiles and non-volatiles. Further, pyrolysis oil has high acidity, which typically leads to corrosion problems, substantial water content, and high oxygen content.
Wood-based pyrolysis oil is the product of pyrolysis of wood or forest residues and it contains typically carboxylic acids, aldehydes, ketones, carbohydrates, thermally degraded lignin, water, and alkali metals. The oxygen-containing compounds (typically 40-50 wt-%) and water (typically 15-30 wt-%) make pyrolysis oils chemically and physically unstable. Although pyrolysis oils have higher energy density than wood, they are acidic (pH˜2) and incompatible with conventional fuels. Furthermore pyrolysis oils have high viscosity and high solid content. Poor stability and high acidity are one of the key problems in utilizing the pyrolysis oil or storing for longer periods.
Due to its instability bio-oil is rapidly transformed to semisolid and gradually solid material, which is difficult to store or use for any further purposes. Thus, according to present practice it is necessary to process the bio-oils rapidly further in order to avoid the problems relating to stability.
Refining of bio-oils and particularly pyrolysis oil to provide fuel or fuel components is often very challenging due to the complex mixture of components of said bio-oil. For example pyrolysis oil typically consists of more than 200 identified compounds, which require very different conditions for converting them further to fuel components or precursors to fuel. Often this is carried out by hydroprocessing said pyrolysis oil over a hydrogenation catalyst in the presence of hydrogen. Since pyrolysis oil typically contains up to 50 wt % of oxygen, complete removal oxygen requires a substantial amount of hydrogen, even up to 1000 L/kg pyrolysis oil. The obtained light components are turned into gaseous products (hydrogen, methane, ethane, etc.) and heavy components are turned into coke and heavy oil. The heavy oil mixture needs further refinement to produce fuel fractions and this procedure requires high amounts of hydrogen and typically various different catalysts for obtaining the desired products.
CN 102643687 A suggests adding methanol in an amount from 1 to 21 wt % to bio-oils for improving stability, whereby viscosity can be reduced and the increase of water content in pyrolysis oil can be prevented.
WO 2012/061005 A2 teaches a process for converting pyrolysis oil to hydrocarbon fuels where pyrolysis oil is contacted with a feed comprising one or more alcohol species in a reactor to form an alcoholysis product, and contacting said alkoholysis product with a hydrotreating catalyst in the presence of hydrogen.
WO 2010/099058 A2 relates to a process for modifying the content of pyrolysis oil where pyrolysis oil vapor is treated with an atomized alcohol or amine under conditions allowing condensation between the carbonyl containing component and the alcohol or amine, and condensing the pyrolysis oil vapor and reaction product to form a pyrolysis oil product having an increased ester or amide content.
Despite the ongoing research and development relating to bio-oils, there is still a need to provide improved processes and methods for converting bio-oils to more valuable components in an efficient and economical way.